A volcano is a geological structure, a feature of the Earth’s crust formed by the eruption of magma and gas onto the surface. The volcano itself is a natural event, a product of tectonic forces deep within the planet. However, when the energy and material from this structure are released, the resulting eruption and its widespread consequences become a natural disaster, defined by the scale of its impact on human society.
Defining a Natural Disaster
An event is classified as a natural disaster when a natural hazard interacts with a vulnerable population, causing severe disruption that overwhelms local ability to cope. Organizations tracking these events, such as the Centre for Research on the Epidemiology of Disasters (CRED), use specific quantitative thresholds for classification. These criteria include the death of ten or more people, the injury or displacement of 100 or more people, or the official declaration of a state of emergency.
The distinction lies between a natural hazard and a resulting disaster. A volcanic eruption occurring in a completely uninhabited area remains a natural event, a geophysical process with no societal impact. A disaster only manifests when the physical event causes extensive damage, loss of life, and requires external aid to manage the crisis and recovery efforts. The resulting destruction from a major eruption meets and often exceeds these benchmarks, categorizing the event as a disaster.
The Volcano as a Geological Feature
A volcano is a vent or fissure in the Earth’s crust that allows molten rock (magma), ash, and gases to escape from a reservoir beneath the surface. These structures are built over time by the accumulation of erupted materials, forming a volcanic edifice. Most volcanoes are situated along tectonic plate boundaries, where plates are either pulling apart (diverging) or colliding (converging).
The type of volcano formed depends on the magma’s composition and the tectonic setting. Steep-sided, conical stratovolcanoes, also called composite volcanoes, typically form over convergent boundaries and are built from alternating layers of lava and ash. Shield volcanoes, with their broad, gentle slopes, are formed by very fluid, low-viscosity lava flows that can travel great distances. Magma chambers beneath these structures build immense pressure, creating the potential for an eruption.
Primary Hazards During an Eruption
The immediate and most destructive output of an explosive eruption is the pyroclastic flow, a superheated, fast-moving mixture of gas, ash, and rock fragments. These flows can travel at speeds exceeding 200 kilometers per hour and reach temperatures of several hundred degrees Celsius, causing total devastation. Nothing organic can survive the intense heat and force of these density currents, making them a primary volcanic hazard.
Lava flows are generally less of a risk to human life because their movement is relatively slow, often progressing at only a few meters per hour. However, they cause complete destruction of infrastructure and property by burying, burning, and crushing everything in their path. Eruptions also eject ballistic projectiles, or volcanic bombs, which are pieces of rock hurled into the air that can land several kilometers away. Heavy ashfall, consisting of pulverized rock and glass, can accumulate quickly, leading to the collapse of building roofs under the weight of the material.
Secondary and Long-Term Consequences
Lahars
The destructive effects of an eruption extend far beyond the immediate blast zone through secondary hazards like lahars, which are fast-moving volcanic mudflows. Lahars are created when water, often from melted snow, glacial ice, or heavy rainfall, mixes with loose volcanic ash and debris on the slopes of the volcano. This slurry can rush down river valleys, traveling long distances and destroying communities far from the volcanic vent.
Atmospheric Effects
Volcanic gas emissions, particularly sulfur dioxide, can cause health hazards both locally and globally. Sulfur dioxide reacts with atmospheric water vapor to form fine sulfate aerosols, which can be transported over thousands of kilometers. These aerosols can cause acid rain and, when injected into the stratosphere by a large explosive eruption, can reflect incoming solar radiation back into space. This reflective layer can lead to a temporary cooling of the Earth’s surface temperature for a period of one to three years, as occurred after the 1991 eruption of Mount Pinatubo.
Ashfall Impacts
Fine volcanic ash poses a health risk because its abrasive nature can lead to respiratory problems upon inhalation. Continued exposure to crystalline silica in the ash may contribute to the development of silicosis, a severe lung disease. Ash deposition can also contaminate water sources, disrupt utility services, and cause widespread crop failure, leading to lasting economic and societal impacts.